In sport and commercial fishing, depth finders are often used to determine the depth of schools of fish. The depth finders are also used to provide the fishermen with a sense of the bottom profile so as to pay-out enough line so as to fish just above the water bottom. Such depth or fish finders display only what is beneath the boat as it is steered by the operator.
In addition to the depth finders, many large commercial fishing vessels also use autopilots to steer the vessel along a course or heading. The autopilots automatically steer the vessel in response to signals received from fixed navigational aids such as longshore radio and navigation (LORAN) stations, global positioning satellites (GPS) systems, or magnetic compasses. These navigational aids steer the boat without regard to conditions existing beneath the vessel.
In at least one instance, an autopilot system utilizes a pair of ultrasonic transducers mounted transversely to the keel of the vessel and directed downwardly to transmit ultrasonic pulses and receive return pulses reflected from the floor of the body of water. Time durations between transmitted and corresponding received pulses are measured to determine floor or channel depth below the port and starboard sides of the vessel. The depths are digitally encoded and supplied to a comparator which generates a signal indicating whether deeper water is port or starboard of the vessel. The comparator signal is supplied to a display to instruct the navigator whether to steer port, starboard, or to maintain course to guide the vessel over the deepest portion of the channel. The comparator signal may also be supplied to the rudder servo to automatically steer the vessel over the deepest portion of the channel.
Doppler systems have been used where two or more beams of radiant energy from the vessel are transmitted at an angle to the earth's surface and portions of the energy are received and processed after reflection from the surface of the earth. The radiant wave energy is transmitted from and received at the two stations spaced as far as practical along the longitudinal axis of the vehicle, the beams of radiant energy being directed parallel to each other at right angles to such axis and at a suitable declination. As the vehicle moves over the surface of the earth, any component of motion at right angles to the longitudinal axis causes the frequency of the received energy to deviate from that of the transmitted energy. By measuring and subtracting the frequency shift at the two stations, the resulting difference is proportional to the velocity and direction of rotation of the vehicle about a vertical axis. zero difference indicating no rotation about the vertical axis. The net velocity can be converted into angular motion of the vehicle about a vertical axis by using the known distance between the transducers.
A primary disadvantage with the autopilots relying on fixed navigational aids is that they are designed to track along a specific route. While the surface track of the vessel may be constant, the track of the vessel above the water bottom may be over irregular and extremely varying water depths. For sport and commercial fishing, it is desired that the boat track above a substantially constant water depth which may well indeed have an irregular or circuitous path. While at least two of the above systems utilize ultrasonic transducers, one system is constantly searching for the deepest portion of the channel in which to steer the vessel, while the other is using Doppler or phase shifts in the ultrasonic signals to indicate which way the vessel has moved. Each of the above systems is extremely complicated and requires at least two transducers to perform the task.